Download protecting rechargeable li-ion and li-polymer batteries

PROTECTING
RECHARGEABLE LI-ION AND
LI-POLYMER BATTERIES
in Consumer Portable Electronics
Littelfuse offers designers many different protection devices to choose
from in an array of form factors and device characteristics that meet the
needs of their particular design.
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PROTECTING
RECHARGEABLE LI-ION AND
LI-POLYMER BATTERIES
in Consumer Portable Electronics
As sleeker designs and thinner portable consumer electronics, such as smart phones, tablets and
other advanced handheld electronics, become increasingly popular, Lithium-ion and LithiumPolymer batteries, known collectively as Li-batteries, have become the “go-to” power sources of
choice in these applications. As battery technology and form factors for consumer devices expand
beyond traditional cylindrical cells, Li-batteries are in increasing demand due to their high energy
density, small form factors and design flexibility. These batteries, in turn, require ever-smaller circuit
protection devices to help provide robust protection in thinner, lower-profile and more compact
portable products.
Need for Battery Protection
Li-battery
faults
Batteries in Cellular Telephones”; and IEC/EN 60950 and
caused by external shorts, runaway charging conditions
packs
are
particularly
sensitive
to
IEC 62133 specifications.). Moreover, certain end product
and abusive overcharging that can result in potentially
applications require that the power output of a battery be
damaging overcurrent and overtemperature conditions. The
limited to reduce the risk of device failures. The Limited
overcharge, deep-discharge, or short circuit conditions that
Power Source (LPS) Test described in UL2054 is used
create heat can cause a Li-battery cell to bloat, rupture, or
to determine whether a cell or battery is suitable in such
experience other issues.
applications where safety issues may otherwise exist.
Although internal cell failures are less common, an adverse
This application note discusses the need for protecting Li-
event may affect any of the complex electronics on the
batteries against short circuit conditions and shows how
battery pack’s PCM (Protection Control Module), such as the
devices from Littelfuse’s business unit, Littelfuse, can help
fuel gauge or charge controller. Because these components
designers achieve robust and safe battery solutions. As
are vulnerable to these events, Li-cells using PCMs require
a pioneer of polymeric positive temperature coefficient
many levels of protection against overcharge shutdown,
(PPTC) resettable devices, Littelfuse has developed several
over-discharge shutdown, overtemperature shutdown, and
material platforms for protecting battery applications. Each
overvoltage/undervoltage lockout of a cell that may lead to
offers different performance characteristics and a range
thermal runaway and possibly failure.
of thermal cut-off, or activation, temperatures PolySwitch
PPTC resettable devices offer form factors including strap,
Organizations such as UL, IEC and IEEE have enforced safety
disc, surface-mount and weldable/reflowable products. The
regulations and established test requirements for Li-ion and
company’s MHP (metal hybrid PPTC) technology offers
Li-Polymer packs to demonstrate their resilience to both
a resettable compact device for high-rate-discharge Li-
short circuit and overcharge events. (For additional details
Polymer and prismatic cell applications. Littelfuse also
refer to UL2054, “Standard for Household and Commercial
offers non-resettable chip fuses in sizes ranging from 0402
Batteries”; IEEE 1725-2006 “Standard for Rechargeable
to 1206.
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
Short Circuit Conditions
An unprotected battery cell or pack can deliver a very high
PolySwitch PPTC Resettable Devices vs.
Traditional Solutions
current when it is “hard shorted” by a low-resistance element.
During a short circuit fault, Littelfuse’s PolySwitch PPTC
In this case, power dissipated in the battery cell’s internal
resettable device rapidly heats up due to the excess current.
impedance can lead to a rise in cell temperature. The severity
As it nears trip temperature, the device increases in resistance
will depend on the pack’s thermal characteristics and the
by several orders of magnitude and limits the fault current
battery cell chemistry. Additionally, accidental short circuits
to a low level. When the fault condition is removed and
can occur when a metal object, such as a keychain, bridges
the power is cycled, the device cools and returns to a low-
the exposed terminals of the battery cell/pack. These short
resistance state. If the fault is not cleared and the power is not
circuits can increase temperatures to levels high enough
cycled, the device will remain latched in the high-resistance
to damage the cell, other components or surrounding
state. During a typical overcharge fault, cell temperature rises
materials. At a minimum, pack performance can deteriorate
when excessive voltage across the fully charged cell causes
and, with some packs, thermal runaway may occur, and can
chemical degradation of cell components.
result in damage. If an unprotected pack is “soft shorted”
by an element with even a small amount of resistance, (e.g.,
When a PolySwitch PPTC device is included in a circuit, as
a few hundred milliohms), the potential problem changes
the cell temperature rises, the temperature of the PolySwitch
from being power dissipated in the cell to power being
device increases accordingly and less current is required to
dissipated in the shorting element. Tests have shown that
trip the device. PolySwitch PPTC devices are often used to
the resistive shorting element can reach temperatures in
replace bimetal or thermal fuse protectors since traditional
excess of 600°C during this type of event, which may result
bimetals often result in bulky, high-cost protection solutions.
in ignition of adjacent combustible materials.
Bimetals normally do not latch in the protected position
during a fault condition, which may result in battery pack
Overcharge Conditions
fault and battery cell damage.
Unlike
resettable
PolySwitch
PPTC
devices,
one-shot
Individual battery chemistries require specific charging
secondary overcurrent protectors, such as thermal fuses, are
profiles to optimize performance and minimize safety issues.
difficult to set at the low temperatures required for charge
If this profile is not met, an overcharge condition may occur.
protection and may trip at high ambient temperatures. Since
A battery pack overcharge condition is most often caused by:
they do not reset, they can cause an otherwise functional pack
to be disabled, which can result in unnecessary field returns.
• A runaway charging condition in which the charger fails
to stop supplying current to the pack once it is fully
Low-temperature PolySwitch PPTC devices help provide
charged. This is typically caused by a charger fault.
overtemperature protection in addition to overcurrent
• Abusive charging that occurs when the pack is charged
protection. The device’s resettable functionality provides
under the wrong conditions by an incorrect or faulty
that nuisance tripping caused by exposure to high storage
charger. The most likely cause of this condition occurs
temperatures, such as leaving a cell phone inside a vehicle
when a consumer uses an aftermarket or non-compatible
on a hot day, does not permanently disable the pack.
charger. Product reliability or safety issues may arise when
using some aftermarket products due to the proprietary
nature of cell chemistries and charger designs.
PolySwitch Devices for Li-Battery Protection
Battery cell overcharge can result from an overcurrent or
Li-packs typically include ICs capable of detecting and
overvoltage condition or a combination of both. If current or
implementing an overvoltage lockout, undervoltage lockout,
voltage is allowed to exceed prescribed values, a significant
overtemperature protection and overcurrent protection. ICs
rise in cell temperature may result. During a typical
and MOSFETs are often used as the primary pack protection
overcharge fault, the cell temperature rises when excessive
in conjunction with a fuel gauge device to track the battery
voltage across the fully charged cell causes chemical
cell capacity, state-of-charge (%), run-time to empty
degradation of the cell components.
(minutes), battery voltage (mV) and temperature.
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
A PolySwitch PPTC device placed in series with the battery
helps provide a second level of protection in the event of
a control circuit malfunctioning (Figure 1). Although the
semiconductor circuitry is considered reliable, there are
conditions under which failure of the primary protection
may occur, including excessive electrostatic discharge, high
temperature or oscillation during a short circuit condition. In
these cases, the PolySwitch PPTC device helps provide cell
overtemperature protection on charge and discharge, as well
as redundant overcurrent protection. When a PolySwitch
PPTC device is included in the circuit, the temperature of the
device increases accordingly as the cell temperature rises
and less current is required to trip the device.
Figure 2. PolySwitch strap devices offer space-saving solution for protecting
battery cells.
A wide selection of PolySwitch PPTC products is offered for
The evolution of Littelfuse’s strap devices has progressed to
Li-battery protection. The PolySwitch family includes devices
lower resistance, smaller form factors and increased thermal
offering a range of thermal cutoff (activation temperatures)
protection, as shown in Figures 3 and 4.
from 85°C to 125°C. The PolySwitch PPTC device’s low
resistance helps meet the battery pack’s resistance budget
requirements, and its low trip temperature helps provide
protection against thermal runaway in case of an abusive
overcharge. PolySwitch PPTC resettable devices are also
available in a variety of form factors and current ratings.
SRP
VTP
LR4
VLR
VLP
MXP
Figure 3. A wide range of PolySwitch strap devices are available for specific
pack requirements.
Figure 1. PolySwitch PPTC device in a typical protection circuit for a Li-ion or
Li-Polymer cell.
PolySwitch Strap Products:
Installation Method – Spot Weld
PolySwitch strap products offered by Littelfuse include
the SRP, LR4, VTP, VLR, VLP, MGP and MXP families. Strap
devices, which come in a flat, tab-like form factor, can be
incorporated into cylindrical based packs, prismatic cells
or even pouch packs and can be applied to specific battery
chemistries or usage profiles. Their installation method is
to be spot welded to cells or straps in the battery pack
(Figure 2).
Figure 4. PolySwitch strap device enables current to be interrupted at different
ambient temperatures.
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
The PolySwitch low-resistance (low rho) MXP strap device,
shown in Figure 5, incorporates conductive metal particles
to achieve lower resistance than traditional carbon blackfilled PPTC devices. The MXP device is 88% smaller than
the prior-generation VTP strap device (Figure 6), while also
providing approximately the same hold current at 60°C.
Regardless of the pack chemistry, device hold current is
selected on the basis of the maximum average charge or
discharge current and takes into account the maximum
operating temperature. The form factor will depend on the
available space within the pack. PolySwitch PPTC strap
devices with activation temperatures (thermal cutoff) from
85°C to 125°C are offered in a wide range of custom and
PolySwitch Surface-Mount Products:
Installation Method – Reflow Solder
PolySwitch surface-mount products are well suited for
battery PCMs since their smaller size helps save board
space and eases design complexity. The nano, micro, and
miniSMDxxxx and the new nano, micro, and miniSMDxxxLR
(low resistance) devices, for example, offer low profiles and
small form factors that can be reflow soldered. While many
surface-mount PPTC devices are already low resistance, the
lower resistance of the miniSMDxxxLR devices help maintain
the system impedance budget.
standard configurations.
PolySwitch L-Tab Devices: Installation
Methods – Reflow Solder and Spot Weld
The PolySwitch L-Tab device helps provide a weldable and
reflowable/solderable device. With operating currents up
to of 4A at room temperature, they are suitable for use
for battery protection in high-performance tablets. They
also offer ultra-low resistance to help maintain the system
impedance budget. Locating protection circuitry in close
proximity to the cell helps eliminate the need for long metal
interconnects and helps improve thermal sensing (Figure 7).
The L-Tab device can be soldered onto the battery PCM and
Figure 5. The low rho MXP strap device and surface-mount device are
placed under the PCB. (The MXP Strap length and configuration can also be
customized per customer requirements.)
the device’s L-shaped tab/terminal can be directly welded
to the battery cell tab, therefore providing cost savings.
Additionally, its “L” shape assists in reducing manufacturing
steps when the PCM is folded into the pack.
Figure 6. Compared to the prior-generation VTP device, the PolySwitch MXP
device has lower resistance in a smaller form factor.
Figure 7. PPTC L-Tab devices occupy the same space as the existing battery
terminal block.
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
PolySwitch Disc Products:
Installation Method – Press Fit
A PolySwitch disc device is a bare disc made of PPTC
material that is placed in a Lithium-ion 18650 cylindrical cell
header to help protect cells during shipping and handling
prior to assembly in packs (Figure 8). The disc device also
helps provide protection for cells that are sold individually
as AA and AAA-sized, non-rechargeable lithium batteries
for consumer use. Each disc device is custom designed for
the cell it will be used with.
Figure 8. A PolySwitch disc device’s placement on an 18650 battery cell.
MHP-TA Devices:
Installation method – Spot Weld
The MHP-TA devices offer a space-saving solution for
protecting higher energy Lithium-polymer and prismatic
battery pack applications such as ultra-thin notebooks
and
tablets.
These
resettable
thermal
cut-off
3.85mm
(TCO)
devices consist of a PolySwitch PPTC device in parallel
with a bimetallic protector. They activate thermally at
temperatures from 72°C to 90°C, while also offering a
high withstand voltage and high hold current. Available in
an ultra-low-profile (L: 5.8mm x W: 3.85mm x H: 1.15mm)
1.15mm
5.8mm
package (Figure 9), their benefits include their extremely
low resistance, ability to open thermally, ability to help
provide latched protection and their resettability. The
MHP-TA device eliminates the non-latching properties of
Figure 9. MHP-TA device targets thinner Li-battery cell applications such as
ultra-thin notebooks.
traditional bimetals because the built-in PPTC keeps the
bimetal contacts latched open during a fault condition.
Surface-Mount Fuses:
Installation Method – Reflow Solder
Littelfuse surface-mount chip fuses can be used for
overcurrent protection in applications where non-resettable
protection is desired. These devices are offered in surfacemount package sizes ranging from 0402 to 1206. Most
safety standards limit the maximum fuse current rating to
five amps. When a failure occurs that clears a fuse, the pack
will be permanently disabled. Fuses, rather than resettable
PolySwitch PPTC devices, are suitable for applications where
a permanent shutdown is desired whenever an overcurrent
fault occurs.
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Protecting Rechargeable Li-Ion and Li-Polymer Batteries
Selecting Circuit Protection Devices
Table 1 shows a selection of Littelfuse PolySwitch devices
The protection requirement is cell chemistry-dependent
that are suitable for Li-battery protection: PolySwitch PPTC
and precise protection requirements should be obtained
devices (strap, surface-mount, disc, L-Tab), as well as the
from the cell manufacturer. Recommendations from device
MHP-TA device and surface-mount fuses.
manufacturers are useful in narrowing protection options
and benchmarking other pack protection schemes may
When adding protection devices, battery pack designers
help provide a good lead for further investigation. However,
must decide what level of protection is required for each
specific testing of each protection option is the best way to
application. A system test should be used to determine
evaluate its effectiveness.
whether or not a specific protection device is appropriate.
Battery
Protection
Device
Strap
Surface-Mount
MHP-TA
L-Tab
Fuse
Disc
Installation
Method(s):
Spot Weld
Reflow Solder
Spot Weld
Reflow Solder,
Spot Weld
Reflow Solder
Press-Fit
SRP, LR4,
VTP,
VLP, VLR
SMD, miniSMD,
microSMD,
nanoSMD,
picoSMD,
femtoSMD
MHP-TA,
MHP-TAM
0402SFx,
0603SFx,
1206SFx
Contact
Littelfuse
for more
information
Product
Family
low rho
MGP, MXP
Contact
Littelfuse
for more
information
low rho
miniSMDLR,
microSMDLR,
nanoSMDLR
Table 1. Selection of Littelfuse PolySwitch products for battery applications.
Summary
Battery applications designers must respond to the trend toward more space-efficient battery packs that require eversmaller protection devices. Littelfuse PolySwitch offers them many different protection devices to choose from in an array of
form factors and device characteristics that meet the needs of their particular design.
Notice:
Information furnished is believed to be accurate and reliable. However, users should independently evaluate the suitability of and test each product selected for their own
applications. Littelfuse products are not designed for, and shall not be used for, any purpose (including, without limitation, military, aerospace, medical, life-saving, lifesustaining or nuclear facility applications, devices intended for surgical implant into the body, or any other application in which the failure or lack of desired operation of the
product may result in personal injury, death, or property damage) other than those expressly set forth in applicable Littelfuse product documentation. Warranties granted
by Littelfuse shall be deemed void for products used for any purpose not expressly set forth in applicable Littelfuse documentation. Littelfuse shall not be liable for any
claims or damages arising out of products used in applications not expressly intended by Littelfuse as set forth in applicable Littelfuse documentation. The sale and use of
Littelfuse products is subject to Littelfuse Terms and Conditions of Sale, unless otherwise agreed by Littelfuse.
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